"The idea that Londoners currently drink water that has passed through at least seven other people is a myth."

I read that to my 12-year-old grandson this morning and he immediately replied: "That's got to be wrong! Everyone knows it happens."

See how the comforting power of an urban myth can be destroyed by just a casual, annoyingly factual, statement.

Next, I thought, they'll be telling us that there is water on the moon. And I turn to The Guardian to find that not only is there an abundance of water, , it is the same water as that on Earth. But surely it will have passed through at least seven green aliens?